1. Field of the Invention
The present invention relates to a mass spectrometer and more particularly to a double focussing mass spectrometer.
2. Description of the Prior Art
To mass-analyze ions having an energy spread with high resolution, a double focussing mass spectrometer is commonly used. With this type of spectrometer, ions having the same mass number are focussed at a single position even if they differ from one another in kinetic energy. Conversely, ions having the same energy as those ions mentioned but having different mass numbers cannot be focussed at the above said position. This focussing action is attained by the combination of an electrostatic and a magnetic field respectively having the following actions; the electrostatic field has an energy converging action in that ions are dispersed and focussed in accordance with their energies while the magnetic field possesses a directional converging action in that ions are dispersed and focussed in accordance with their mass number. In other words, ions emitted from an ion source are lead into an electrostatic field and if some of the ions have the same energy, they are focussed at a first predetermined focus position. The focussed ions are then led into a magnetic field and of all the focussed ions a group of ions having a certain mass number are focussed at a second predetermined focus position while the other ions not belonging to the group are not focussed on the second focus position. On the other hand, of the ions which the electrostatic field failed to focus at the first focus position, some having the same mass number as those focussed at the second focus position are also focussed at the second focus position by means of the magnetic field and the other having different mass numbers are diverted from the second focus position. Accordingly, if a collector slit is disposed at the second focus position, the ions passing through the slit can be received by an ion detector.
It is very preferable for manufactures as well as users to reduce the size and cost of such a double focussing mass spectrometer without adversely affecting its function.
It may be considered that the simplest way to reduce the size diminishes the angles of deflection by electric and magnetic fields, because this artifice leads to the reduction of the spaces for the electrostatic and magnetic fields. However, if the angles of deflection are decreased, the focus position becomes remoter and the linear portions of the ion trajectory also become longer, so that the decrease in the spaces for the electrostatic and magnetic fields is canceled by the increase in the length of trajectory, the size of the spectrometer remaining substantially unaltered. Moreover, the remoter is the focussing position, the longer is the distance from the ion source to the electrostatic field. In this case, therefore, the utility factor of the ions produced in the ion source is lowered, which results in the degradation of the sensitivity of the mass spectrometer.